Yadollah Khoshbakht - Nutrition and Food Security Research Center, Shahid Sadoughi University of Medical Sciences, Yazd, Iran.
Amin Salehi-Abargouei - Nutrition and Food Security Research Center, Shahid Sadoughi University of Medical Sciences, Yazd, Iran.

Background and Aim: The association between vitamin D and attention deficit hyperactivity disorder (ADHD) has been proposed by several researchers in recent years; however, the investigations have led to inconsistent results. The present study was conducted to summarize the published observational data on the relationship between vitamin D status and the likelihood of ADHD. Methods: Online databases including PubMed, ISI web of Science, Google Scholar, and Scopus were checked up to June 2017 for relevant observational studies. Out of 2770 retrieved articles, thirteen observational studies (nine case-control or cross-sectional and four prospective studies) were eligible to be included in the systematic review and meta-analysisResults: Analysis of 10334 children/adolescents attended nine case-control or cross-sectional studies revealed that children with ADHD have lower serum levels of 25-hydroxyvitamin D [25(OH)D] compared with healthy children [weighted mean difference=-6.75 ng/ml, 95% confidence interval (CI): 9.73 to 3.77, I2 = 94.9%]. Five case-control studies reported odds ratio for developing ADHD based on vitamin D status; the meta-analysis of their data revealed that lower vitamin D status is significantly associated with the likelihood of ADHD (odds ratio=2.57, 95% CI: 1.09-6.04, I2 = 84.3%). Furthermore, the meta-analysis of prospective studies conducted in 4137 participants indicated that perinatal sub-optimal vitamin D levels are significantly associated with higher risk of ADHD in later life (relative risk= 1.40, 95% CI: 1.09-1.81, I2 = 0.0%). Conclusion: The present review provides evidence supporting the relationship between vitamin D deficiency and ADHD. Further prospective cohort studies and community-based intervention trials are highly recommended to better elucidate the causal association.

Vitamin D; Attention deficit disorder with hyperactivity; Children; Adolescents; Systematic review and Meta-analysis